The Heating of Solar Coronal Loops by Alfvén Wave Turbulence

Abstract

In this paper we further develop a model for the heating of coronal loops by Alfvén wave turbulence (AWT). The Alfvén waves are assumed to be launched from a collection of kilogauss flux tubes in the photosphere at the two ends of the loop. Using a three-dimensional magneto-hydrodynamic (MHD) model for an active-region loop, we investigate how the waves from neighboring flux tubes interact in the chromosphere and corona. For a particular combination of model parameters we find that AWT can produce enough heat to maintain a peak temperature of about 2.5 MK, somewhat lower than the temperatures of 3 -4 MK observed in the cores of active regions. The heating rates vary strongly in space and time, but the simulated heating events have durations less than 1 minute and are unlikely to reproduce the observed broad Differential Emission Measure distributions of active regions. The simulated spectral line non-thermal widths are predicted to be about 27 km s −1 , which is high compared to the observed values. Therefore, the present AWT model does not satisfy the observational constraints. An alternative "magnetic braiding" model is considered in which the coronal field lines are subject to slow random footpoint motions, but we find that such long period motions produce much less heating than the shorter period waves launched within the flux tubes. We discuss several possibilities for resolving the problem of producing sufficiently hot loops in active regions.

Exploring the damping of Alfvén waves along a long off-limb coronal loop, up to 1.4 R⊙

“…Recently, van Ballegooijen et al (2017) developed an Alfvén wave turbulence model for the heating of coronal loops. Their model was able to reproduce a coronal loop with a temperature about 2.5 MK and predicted non-thermal velocity to be about 27 km s −1 .…”

IRIS Observations of Short-term Variability in Moss Associated with Transient Hot Coronal Loops

“…Though significant progress has been made (e.g., Klimchuk 2006;Parnell & De Moortel 2012;Reale 2014;Testa et al 2015;Klimchuk 2015) the details of the coronal heating are still poorly understood. Different processes at work in the solar atmosphere are viable candidate as responsible for coronal heating, including dissipation of magnetohydrodynamic Alfvén waves (e.g., van Ballegooijen et al 2011van Ballegooijen et al , 2017, dissipation of magnetic stresses built through random as well as to non-equilibrium ionization which further decreases the emission at high temperature.…”

Non-thermal line broadening due to braiding-induced turbulence in solar coronal loops

“…Instead, a rather hot background already exists in the initial condition. On the other hand, reduced MHD models that do self-consistently energize the coronal loops through Alfvénic waves do not match the observed temperatures and non-thermal broadening simultaneously: if the non-thermal broadening is matched to observations then temperatures are too low, while in simulations that reproduce the observed temperatures the non-thermal broadening is too high (Asgari-Targhi et al 2014;van Ballegooijen et al 2017). As noted above, typical observed non-thermal broadening is in the range 15-20 km s −1 , while in these models typically w nth ∼ 27 km s −1 .…”